763 NUCLEAR ENGINEERING AND TECHNOLOGY, VOL.38 NO.8 DECEMBER 2006 INVESTIGATION OF DRAG REDUCTION MECHANISM BY MICROBUBBLE INJECTION WITHIN A CHANNEL BOUNDARY LAYER USING PARTICLE TRACKING VELOCIMETRY YASSIN A. HASSAN * and C. C. GUTIERREZ-TORRES Department of Nuclear Engineering Texas A&M University College Station Texas 77843-3133 * Corresponding author. E-mail : y-hassan@tamu.edu Received November 14, 2006 1. INTRODUCTION Drag reduction is a complex phenomenon that has been studied for several years. The benefits of a success in drag reduction control are enormous. Reduction in drag can increase range or speed in transportation systems, reduce the energy consumption in pumping system, improve systems efficiency, and decrease fuel consumption with the indirect consequences of cost savings and decrease in pollutants emission. In addition, the interaction of microbubble with the flow in the near-wall turbulence structures in turbulent boundary layer is an important complex phenomenon. It is responsible for fluid mixing and increased heat transfer from the nuclear fuel rods to the coolants during the subcooled boiling processes. The control of these processes requires a through understanding of underlying physical mechanism. As an example, the departure of nucleate boiling predictions are less than satisfactory despite the studies and great interest they have generated over the years. In this study, isothermal microbubble boundary layer dynamics will be investigated to shed some the ubiquitous structural features in the near-wall region of turbulent boundary layers. Polymer additives injection, surfactants injection, riblets, wall oscillations, traveling waves, blowing, and microbubbles injection within the boundary layer are among methods, which have been studied to achieve understanding of the drag reduction mechanism. Injection of microbubbles in the inner zone of the boundary layer to achieve drag reduction has been investigated since the sixties and early seventies. The environment friendly characteristics of this technique in achieving drag reduction make it an attractive method. Recently, efforts have been sparked again with a promise of achieving the goal of saving energy and mitigating the impact on the environment. McCormick and Bhattacharyya [1] reported one of the first microbubble experimental results using electrolysis to produce hydrogen microbubbles on the hull of a submersible axisymetric body. Drag reduction values as high as 30% were obtained. Reported results showed that the amount of drag reduction depends on the speed and the rate of hydrogen production. A decrease in the Reynolds stresses was observed. Madavan et al. [2] carried out a numerical investigation of the phenomenon in an effort to clarify the effect of the presence of the microbubbles on the physical properties values (as density and viscosity) of the fluid in the boundary layer. The main conclusions presented in their work were that the microbubbles’ presence in the boundary layer affects the turbulent structure by altering the local effective Injection of microbubbles within the turbulent boundary layer has been investigated for several years as a method to achieve drag reduction. However, the physical mechanism of this phenomenon is not yet fully understood. Experiments in a channel flow for single phase (water) and two phase (water and microbubbles) flows with various void fraction values are studied for a Reynolds number of 5128 based on the half height of the channel and bulk velocity. The state-of-the art Particle Tracking Velocimetry (PTV) measurement technique is used to measure the instantaneous full-field velocity components. Comparisons between turbulent statistical quantities with various values of local void fraction are presented to elucidate the influence of the microbubbles presence within the boundary layer. A decrease in the Reynolds stress distribution and turbulence production is obtained with the increase of microbubble concentration. The results obtained indicate a decorrelation of the streamwise and normal fluctuating velocities when microbubbles are injected within the boundary layer. KEYWORDS : Drag Reduction, Microbubbles, Particle Tracking Velocimetry, Local Void Fraction